What is a REST API and why your business needs one

Today's businesses run on dozens of different applications and platforms: an ERP for accounting and resources, a CRM for customer relationships, an ecommerce platform for online sales, marketing tools, logistics systems and much more. The real challenge isn't adopting these tools. It's getting them to talk to each other efficiently. That's where APIs come in.

APIs (application programming interfaces) are the invisible glue that holds a company's digital systems together. Without them, every application would sit in its own silo, and employees would have to re-enter data by hand, export and import CSV files and deal with constant inconsistencies. With them, information flows automatically between systems, errors disappear and your team gets time back for higher-value work.

What is an API?

An API (application programming interface) is a set of rules and protocols that lets two applications communicate with each other. It acts as an intermediary: it receives requests from one application, translates them into a format the other one understands and returns the response.

A simple analogy: think of a restaurant. You (the customer, or the application requesting data) don't walk into the kitchen (the server or database). Instead, you tell the waiter (the API) what you want. The waiter passes your order to the kitchen and brings back your dish. The waiter is the API: it defines what you can order, how to order it and in what form you'll receive it.

In technical terms, an API exposes a set of endpoints (URLs) that you can send requests to in order to retrieve data, create records, update information or delete resources. Each endpoint has a specific, documented purpose.

REST vs. SOAP: two different approaches

When it comes to web APIs, the two main approaches are REST and SOAP. SOAP was the standard for many years, but REST has become the dominant choice thanks to its simplicity and flexibility.

SOAP (Simple Object Access Protocol)

SOAP is a protocol that uses XML to exchange messages between systems. Its format is very strict: every message must follow a specific structure (envelope, header, body) and is described by a WSDL file (Web Services Description Language). It was very popular in enterprise environments throughout the 2000s.

Its main strengths are reliability (it has built-in security and transaction mechanisms) and strict typing (it defines exactly what data is sent and received). However, its verbosity (XML messages are heavy) and its implementation complexity have made it lose ground to REST.

REST (Representational State Transfer)

REST isn't a protocol but an architectural style, defined by Roy Fielding in his doctoral dissertation in 2000. It builds on the principles of HTTP and uses its native verbs (GET, POST, PUT, DELETE) to perform operations on resources identified by URLs.

REST APIs are lighter than SOAP APIs, can return data in several formats (JSON, XML, HTML) and are easier to understand and consume. JSON has become the de facto standard format because it is readable and lighter than XML.

83% of today's public APIs use the REST style. Its simplicity, scalability and compatibility with modern web technologies make it the natural choice for most projects.

HTTP methods in REST

REST uses the methods (or verbs) of the HTTP protocol to define the operation you want to perform on a resource. The four main methods map to the CRUD operations (Create, Read, Update, Delete):

• GET: reads a resource. It doesn't change any data. Example: GET /api/customers/42 returns the data for the customer with ID 42.
• POST: creates a new resource. Example: POST /api/customers with a JSON body containing the new customer's data.
• PUT: replaces an entire existing resource. Example: PUT /api/customers/42 replaces all of customer 42's data with the data sent in the request body.
• PATCH: partially updates a resource. Example: PATCH /api/customers/42 changes only the fields you send and leaves the rest untouched.
• DELETE: removes a resource. Example: DELETE /api/customers/42 deletes the customer with ID 42.

Combined with a consistent, predictable URL structure, these methods make REST APIs intuitive to use. A developer who knows how a REST API is structured can easily work out how to interact with its resources without reading exhaustive documentation.

Authentication and security

Security is critical for any API. A poorly protected API can expose sensitive customer data, financial transactions or confidential company information. The most common authentication mechanisms are:

API keys

The simplest method. Each client receives a unique key (API key) that it must include in every request, usually as an HTTP header or URL parameter. It's easy to implement but has its limits: if the key leaks, anyone can access the API.

OAuth 2.0

The most widely used standard for delegated authentication. It lets an application access a user's resources on another service without knowing their credentials. It's the mechanism behind "Sign in with Google" or "Sign in with Facebook". OAuth 2.0 uses access tokens that expire, plus refresh tokens to renew them.

JWT (JSON Web Tokens)

JWTs are digitally signed tokens that contain encoded information about the user and their permissions. They don't need to be stored on the server (they are stateless), which makes them ideal for distributed architectures and microservices. Every request includes the JWT, and the server validates it by checking the signature.

Security best practices

• Always use HTTPS to encrypt communications.
• Implement rate limiting to prevent abuse and denial-of-service attacks.
• Validate and sanitize all input data to prevent SQL injection and XSS.
• Log every request for auditing and anomaly detection.
• Use CORS (Cross-Origin Resource Sharing) to control which domains can access your API.
• Keep access token lifetimes short and rotate API keys regularly.

API documentation

An API without documentation is like a product without a user manual. Documentation is essential so that developers, both internal and external, can use your API efficiently.

The de facto standard for documenting REST APIs is the OpenAPI Specification (formerly known as Swagger). It describes every endpoint, parameter, response, error code and data model in a format that both humans and machines can read.

Good API documentation should include:

• An overview and the purpose of the API.
• An authentication guide with examples.
• A complete list of endpoints and their HTTP methods.
• Input parameters with data types, allowed values and whether they are required or optional.
• Sample requests and responses for each endpoint.
• HTTP status codes and possible error messages.
• Usage limits (rate limiting) and pagination.
• A test environment (sandbox) where developers can experiment without touching real data.

Business use cases

REST APIs have practical uses in almost every area of running a business. These are the most common use cases:

Connecting your CRM and ERP

Integrating your CRM (where you manage sales opportunities and customer relationships) with your ERP (where you manage accounting, inventory and invoicing) removes duplicate data and ensures the whole organization works with up-to-date information. When a sales rep closes a deal in the CRM, the API can automatically create the order and the invoice in the ERP.

Payment gateways

Payment gateways such as Stripe, PayPal or Redsys (the Spanish card payment gateway) expose REST APIs that let your application process payments, manage subscriptions, issue refunds and query transaction history programmatically. That way, payment collection becomes part of your business workflow, without leaving your application.

Marketplaces

Amazon, eBay, AliExpress and other marketplaces offer APIs to manage your product catalog, sync stock, receive orders and update prices centrally. Instead of managing each marketplace separately, an API integration lets you control everything from a single dashboard.

Logistics and transportation

Carriers such as SEUR, MRW, DHL or Correos (the Spanish postal service) offer APIs to generate shipping labels, schedule pickups, track parcels and calculate rates in real time. Integrating these APIs into your management system fully automates your logistics process.

E-invoicing

As e-invoicing becomes mandatory step by step in Spain, invoicing platform APIs let you generate, send and store electronic invoices automatically, in line with the legal requirements of the Spanish Tax Agency (AEAT).

Webhooks: real-time notifications

REST APIs work on a request-response model (the client asks, the server answers). Webhooks reverse that flow: the server notifies the client when a relevant event occurs.

For example, a payment gateway can send a webhook to your application whenever a payment is completed, a refund is issued or a transaction fails. Your application receives the notification in real time and can act on it: update the order status, send a confirmation email or record the payment in the ERP.

Webhooks are especially useful when you need to react immediately to external events without constantly querying the API (known as polling). They reduce the load on both systems and deliver an almost instant response.

APIs and microservices architecture

A microservices architecture splits a monolithic application into small, independent services, each responsible for a specific function (users, payments, inventory, notifications). These services communicate with each other through REST APIs (or asynchronous messaging).

The advantages of this architecture are significant:

• Independent scaling: you can scale only the service that needs it (for example, the payments service during a Black Friday campaign) without scaling the whole application.
• Independent deployment: you can update one service without affecting the others. This speeds up development cycles and lowers the risk of each release.
• Mixed technologies: each service can be built in the language and framework best suited to its job.
• Resilience: if one service fails, the rest of the application can keep running.

For companies that are growing fast or need to evolve their software quickly, combining microservices and REST APIs provides a solid, future-ready architectural foundation.

REST API design best practices

If your company is going to build its own APIs (for internal use or for third parties), following these best practices will give you a robust, maintainable and easy-to-use API:

1. Use nouns for resources, not verbs: /api/customers instead of /api/getCustomers.
2. Versioning: include the version in the URL (/api/v1/customers) so you can evolve the API without breaking existing integrations.
3. Pagination: for endpoints that return long lists, implement pagination with parameters such as page and limit.
4. Filtering and sorting: let consumers filter and sort results with query string parameters.
5. Correct HTTP status codes: 200 (OK), 201 (Created), 400 (Bad Request), 401 (Unauthorized), 404 (Not Found), 500 (Server Error).
6. Informative error messages: include an error code, a descriptive message and, where possible, a suggested fix.
7. HATEOAS: include links to related resources in responses to make the API easier to navigate.
8. Idempotency: GET, PUT and DELETE operations should be idempotent (produce the same result no matter how many times they run).

Conclusion

REST APIs are the universal language that lets a company's digital systems talk to each other. When businesses rely on dozens of different applications, the ability to connect them reliably and automatically is a decisive competitive advantage.

Whether you want to sync your CRM with your ERP, process payments inside your workflow, manage several marketplaces from one dashboard or build a scalable microservices architecture, REST APIs are the key piece that makes that connectivity possible.

If your company still moves data between systems by hand (CSV exports, copy and paste, double entry), now is the time to explore how an API-based integration strategy can transform your operations and free your team to focus on what really creates value.